Effective placement of self‐centering damage‐free connections for seismic‐resilient steel moment resisting frames

In recent years, significant advancements have been made in the definition of innovative “minimal-damage structures,” chasing the need for more resilient societies against extreme seismic events. In this context, moment resisting frames (MRFs) equipped with self-centering damage-free (SCDF) devices in column bases and beam-to-column joints represent a viable solution to improve structural resilience and damage reduction. However, the extensive use of these devices significantly increases complexity and costs compared to conventional structures, thus limiting their practical application. To overcome this drawback, current research works are focusing on the definition of effective placement for SCDF devices, maximizing their beneficial effect on the seismic response and controlling their impact on the overall structural complexity. Within this context, the present study investigates the influence of the placement of SCDF devices in a steel MRF. An eight-story MRF is designed, and 50 configurations with different locations of SCDF joints are considered. Numerical models are developed in OpenSees, and non-linear static push–pull and incremental dynamic analyses (IDAs) are carried out. The influence of the placement of SCDF devices is assessed by considering residual and peak interstory drifts, residual top story drifts, peak story accelerations, and the total dissipated energy as performance parameters. The results of IDAs for a seismic intensity corresponding to the ultimate limit state (ULS) are analyzed and compared, and fragility curves are successively derived for some relevant configurations. The paper provides insights and observations to understand how including a different number of SCDF BCJs at different stories affects the seismic response

Analisi teorico-sperimentale di collegamenti dissipativi a parziale ripristino di resistenza in strutture intelaiate in acciaio

2009 - 2010Before seismic events of Northridge (Los Angeles, 17 January 1994) and Hyogoken-Nanbu (Kobe, 17 January 1995) MRFs were supposed to be the most reliable seismic resistant systems due to the high number of dissipative zones that are able to develop Before these earthquakes, especially in United States, MRFs were realized, generally, by adopting fully welded connections, which, at the time, were retained to perform better compared to other joint typologies. In addition, the economic advantages deriving from the adoption of field fully welded connections, strongly influenced choices of building owners’ and, as a result, led to the adoption of this joint typology in almost all pre- Northridge steel MRFs. After Northridge earthquake, even though the loss of life was limited, the unexpected amount of damages occurred in structures adopting as seismic resistant system welded Moment Resisting Frames put into question the role played by welded connections on the whole structural behavior. Therefore, after the seismic events, two strategies were identified to improve behavior of fully welded connections. The first one is related to the improvement of the welding technique, usually strengthening the critical area subjected to fracture. The second one is based on the possibility of concentrating the energy dissipation in the beam, reducing the bending resistant area of beams by properly cutting the flanges in a zone close to beam-to-column connection. This weakening approach is commonly called RBS. A new design approach, which has been the subject of many studies in last decades, has gained growing interest in last years. In fact, Eurocode 8 has opened the door to the idea of dissipating the seismic input energy in the connecting elements of beam-to-column joints. It has been recognized that semi-rigid partial strength connections can lead to dissipation a and ductility capacity compatible with the seismic demand, provided that they are properly designed by means of an appropriate choice of the joint component where the dissipation has to occur. In this work, the attention is focused on this last approach. The first part of the work is descriptive and deals with the historical development and, in general, with the seismic behavior of Moment Resisting Frames. In the same chapter general concepts concerning the component method, as introduced by last version of Eurocode 3, are given. Finally, the influence of the joint behaviour on main characteristics of partial strength and/or semi-rigid MRFs is evaluated by properly accounting for existing literature. Third chapter deals with an experimental analysis on the cyclic behaviour of classical partial strength beam-to-column joints. The main scope of the experimental campaign is to show how to control the dissipative behaviour of joints by properly designing the weakest joint component and by over-strengthening the other connecting elements. Therefore, a design procedure is pointed out and the comparison among the results obtained by cyclic tests is presented in terms of energy dissipation capacity. In addition, by monitoring during the experimental tests both the whole joint and the single joint components it is shown that the energy dissipated by the joint is equal to the sum of the energy dissipated by the joint components. This result assures that the first phase of the component approach, i.e. the component identification, has been properly carried out and that interaction between components under cyclic loads is negligible. Chapter 4 represents the continuation of the work carried out in previous chapter. In fact, on the base of the obtained results, the goal is to provide a mechanical cyclic model for the prediction of the overall joint behaviour, starting from existing literature models. Hence, a state-of-the-art review is first presented and then, model employed to set up a computer program devoted to the prediction of the cyclic behaviour of steel beam-to-column joints is shown. In particular, cyclic model adopts Kim & Engelhardt model for shear panel, Cofie & Krawinkler model for Panels in Tension and Compression and Piluso et al. model for the prediction of the T-stub behavior. Finally, in chapter 5, an innovative approach to improve the seismic behavior of bolted beam-tocolumn joints, which are affected by strength and stiffness degradation, is presented. The development of a dissipative device representing the application of ADAS concept to T-stubs, is detailed. First, a mechanical and finite element model able to predict the whole force displacement curve of the so-called hourglass T-stubs are set up. Next, an experimental analysis aiming to compare the hysteretic behaviour and the dissipative capacities of rectangular and dissipative T-stubs is carried out. Finally, as a consequence of the study of the joint component, a further experimental analysis concerning the application of such devices to partial strength beam-to-column joints is presented and the results, in terms of moment-rotation curve and energy dissipation capacity, are discussed and compared to those obtained by the cyclic testing of classical joints.[edited by author]IX n.s

The Design of a Real-Scale Steel Moment-Resisting Frame for Pseudo-Dynamic Earthquake Testing

Background: Forthe reliable prediction of the non-linear response of structures, severe seismic events have proven to be a challenging task. Although much non-linear analysis software exists, the accuracy of the results depends on the assumptions made in the characterization of the members. Typically, the analytical models are calibrated using experimental observations. With this scope, experimental research remains the most reliable mean for the assessment of the seismic performance of structures, and it is crucial to target the development of new analytical models and design methods. Objective: Quasi-static tests can provide information on the non-linear behaviour of subassemblies, but it is often difficult to relate the imposed force or displacement histories to those that might occur during an earthquake. The pseudo-dynamic method combines an on-line computer simulation with experimental information about the tested structure, providing the application of realistic dynamic response histories. In this paper, the preliminary analysis and the design of a pseudo-dynamic testing facility for the experimental study of a real scale two storeys-two bays steel MRF, with classical and innovative joint details, are shown. Methods: Pushover and Incremental Dynamic Analyses carried out with Seismostruct software estimate the forces and displacements expected at each storey for the selected ground motions. These analyses have been performed by varying the structural detail of the beam-to-column connections. Results / Conclusion: In this paper, the analytical prediction of the performance of two bays-two storeys steel frames equipped with different solutions of beam-to-column joints is focused. Based on the performed analyses, it has been recognized that steel frames with partial strength joints can provide satisfactory performance under severe seismic actions provided that the joints are adequately designed

Genetic Algorithm for the optimal placement of Self-Centering Damage-Free joints in steel MRFs

Nowadays' earthquake engineering is coping with the challenging task of providing low-cost seismic resilient structures. Among others, a viable solution for seismic resilient Steel Moment Resisting Frames (MRFs) is based on the use of Self-Centering Damage-Free (SCDF) joints at Column Bases (CBs) and Beam-to-Column Joints (BCJs), ensuring both the energy dissipation capacity and self-centering behavior of the structure. Past studies demonstrated the beneficial effects gained in damage and residual drifts reduction by including SCDF joints at all BCJs and CBs. However, this solution leads to the highest structural complexity, limiting the practical application. Significant improvements can be obtained including a limited number of SCDF BCJs, but there is a lack of generalized recommendations on the number required and their effective placement. In this work, a Genetic Algorithm (GA) is proposed to define the optimal placement of SCDF BCJs in steel MRFs. The GA is implemented in Matlab, and non-linear time-history analyses are performed in OpenSees to calculate the Fitness-Function. The results of the GA are validated against a Brute-Force Approach. An 8-story 3-bays steel MRF and a type of SCDF joint are selected for case study purposes, non-linear Finite Element Models are developed in OpenSees, and the GA is applied. The results show that the proposed GA is an efficient methodology to solve the considered optimization problem

Pseudo‐dynamic testing, repairability, and resilience assessment of a large‐scale steel structure equipped with self‐centering column bases

Recent destructive seismic events have underlined the need for increasing research efforts devoted to the development of innovative seismic‐resilient structures able to reduce seismic‐induced direct and indirect losses. Regarding steel Moment Resisting Frames (MRFs), the inclusion of Friction Devices (FDs) in Beam‐to‐Column Joints (BCJs) has emerged as an effective solution to dissipate the seismic input energy while ensuring a damage‐free behavior. Additionally, recent studies have demonstrated the benefits of implementing similar damage‐free solutions for Column Bases (CBs). In this context, the authors have recently experimentally investigated a Self‐Centering CB (SC‐CB) aimed at residual drift reduction. Previous experimental tests only focused on the response of isolated SC‐CBs under cyclic loads. Conversely, the present paper advances the research through an experimental campaign on a large‐scale steel structure equipped with the proposed SC‐CBs, providing valuable insights into the global structural response and improved repairability. A set of eight Pseudo‐Dynamic (PsD) tests were conducted considering different records and configurations of the structure. The experimental results highlighted the effectiveness of the SC‐CBs in minimizing the residual interstory drifts and protecting the first‐story columns from damage, thus enhancing the structure's resilience. Moreover, the consecutive PsD tests allowed investigating the effectiveness of the reparation process in restoring the seismic performance of the ‘undamaged’ structure. An advanced numerical model was developed in OpenSees and validated against the global and component‐level experimental results. Incremental Dynamic Analyses were finally performed to investigate the influence of the SC‐CBs on the structure's seismic response while accounting for the record‐to‐record variability

Seismic Behavior of Moment-Resisting Frames with Conventional and Innovative Connections

In the last few decades, increasing efforts have been devoted to the development of beam-to-column connections able to accommodate the local ductility demand dissipating, contemporaneously, the seismic input energy. Among the typologies proposed, the so-called RBS (Reduced Beam Section) has gained wide acceptance in the construction market, leading to easy-to-construct and cost-effective solutions. As an alternative, new proposals based on the inclusion of friction devices in beam-to-column joints have recently been made. Such a practice has the merit, in case of destructive events, of exhibiting wide and stable hysteretic cycles concentrating damage in elements that undergo only minor yielding. Both RBS and friction joints have been widely studied, carrying out experimental tests on sub-assemblies investigating their cyclic rotational response. Nevertheless, the available experimental results on full-scale structures equipped with these connections are still quite limited. This is the reason why two experimental campaigns aimed at performing pseudo-dynamic testing of a full-scale two-storey steel building equipped with RBS and friction connections have been planned at the STRENGTH (STRuctural ENGineering Test Hall) Laboratory of the University of Salerno. The first experimental campaign with the structure equipped with RBSs has already been performed; the connections showed higher resistance than expected, and exhibited brittle fracture due to cyclic fatigue. The second campaign has not yet been carried out, but in this paper the blind analysis of the supposed behavior is reported. It is expected that the friction joints allow to dissipate the seismic input energy without any structural damage in the members, but only through the friction pads of the devices, which can be easily replaced at the end of a severe seismic event

Parametric Study and Finite Element Modelling of Damage-Free Self-Centring Column Bases with Different Structural Properties = Studio parametrico e modellazione agli elementi finiti di nodi di base in acciaio ricentranti dotati di differenti proprieta’ strutturali

Negli ultimi anni la ricerca scientifica ha avanzato progressi significativi nello sviluppo di sistemi strutturali innovativi, allo scopo di ottenere un comportamento resiliente nei confronti delle azioni sismiche, inseguendo l’urgente necessità di ridurre i costi ed i tempi di riparazione in seguito ad eventi sismici severi. A tal proposito, i nodi di base autocentranti sono emersi come una soluzione efficace per migliorare le prestazioni sismiche dei telai sismo-resistenti in acciaio, riducendo il danneggiamento locale e conferendo una migliore capacità di ricentraggio all’intero edificio. Tuttavia, sebbene numerose tecnologie siano state concepite, studiate e testate sperimentalmente, solo pochi studi di ricerca hanno indagato le proprietà significative delle connessioni che influenzano il comportamento di tali sistemi. A tal fine, focalizzando l’attenzione su un giunto di base autocentrante precedentemente proposto, studiato e testato dagli autori, il presente studio ha l’obiettivo di eseguire un’analisi parametrica agli elementi finiti per valutare l’influenza di alcuni parametri di progetto sulla risposta globale e locale di tali giunti, tenendo conto degli obiettivi di ottenere un comportamento autocentrante, nonché di ridurre al minimo il danneggiamento delle singole componenti del giunto. A tale scopo, è stato sviluppato un modello agli elementi in ABAQUS e validato rispetto ai risultati sperimentali. Successivamente, tre modelli agli elementi finiti di giunti appartenenti a diversi casi-studio sono progettati e modellati considerando sedici configurazioni per ogni caso, ciascuna caratterizzata da parametri di progetto e proprietà strutturali differenti. L’analisi parametrica fornisce una visione più ampia e completa sulle assunzioni e sui limiti della metodologia di progetto e suggerisce ulteriori raccomandazioni per migliorare i requisiti di progetto di tali giunti

Ottimizzazione del posizionamento di nodi ricentranti in telai momento resistenti in acciaio tramite un algoritmo genetico = Optimal Placement of Self-Centering Joints in Steel Moment Resisting Frames Through a Genetic Algorithm

Nowadays’ earthquake engineering is coping with the challenging task of providing low-cost seismic resilient structures. For Steel Moment Resisting Frames (MRFs) a viable solution is to use Self-Centering Damage-Free (SCDF) devices at Column Bases (CBs) and Beam-to-Column Joints (BCJs), ensuring both the energy dissipation capacity and the self-centering behavior of the structure. Past studies demonstrated the beneficial effects gained in damage and residual drifts reduction through the use of SCDF devices in all BCJs and CBs. However, this solution leads to the highest structural complexity and cost, limiting the practical application of these systems. Significant improvements can be obtained including a limited number of SCDF BCJs, but there is a lack of generalized recommendations on the number required and the effective placement. In this work, a Genetic Algorithm (GA) is proposed to define the optimal placement of a limited number of SCDF BCJs in steel MRFs. The GA is implemented in Matlab, and Non-Linear Time-History Analyses are performed in OpenSees to calculate the Fitness-Function based on residual drifts. An 8-story 3-bays steel MRF and a type of SCDF device are selected as case study and non linear Finite Element Models (FEM) are developed in OpenSees. The GA is applied assuming 16 SCDF BCJs and it is validated through a Brute Force approach. The results show that the proposed GA is an efficient methodology to solve the considered optimization problem. // Uno degli obiettivi principali dell'ingegneria sismica odierna è quello di progettare strutture sismo-resilienti a basso costo e tali da poter essere ampiamente utilizzate su larga scala. Per i telai momento resistenti in acciaio (MRFs), una possibile soluzione consiste nell'utilizzo di dispositivi dissipativi-ricentranti (SCDF) nei nodi trave-colonna (BCJs) e nodi di base (CBs) in modo da garantire sia la capacità dissipativa che il comportamento ricentrante della struttura. È stato dimostrato che l'uso di tali dispositivi in tutti i BCJs e CBs ha effetti benefici nella riduzione dei danni e degli spostamenti residui. Tuttavia, tale soluzione incrementa complessità e costo strutturale, limitando l'applicazione di questi sistemi nella pratica progettuale. Al fine di raggiungere un compromesso, studi precedenti hanno investigato l’utilizzo di un numero limitato di SCDF BCJs dimostrando che tali soluzioni possono fornire miglioramenti significativi in termini di prestazione sismica mantenedo una complessità ridotta. Tuttavia non ci sono ancora raccomandazioni generalizzate rispetto al numero di dispositivi necessario e al loro efficace posizionamento. In questo studio, viene proposto un Algoritmo Genetico (GA) per definire il posizionamento ottimale di un numero limitato di SCDF BCJs in MRFs in acciaio. Il GA è implementato in Matlab ed analisi dinamiche non lineari sono eseguite in OpenSees per calcolare la Fitness-Function basata sui drifts residui. Un MRF in acciaio con 8 piani e 3 campate e un tipo di dispositivo SCDF sono considerati per la definizione di un caso studio. Il GA è applicato assumendo 16 SCDF BCJs e validato rispetto un approccio Brute Force. I risultati mostrano che il GA proposto è una metodologia efficiente per risolvere il problema di ottimizzazione considerato

Pseudo-Dynamic Testing of a Seismic-Resilient Steel Structure Equipped with Damage Free Beam-to-Column Connection and Self-Centring Column Bases = Prove pseudodinamiche su una struttura sismoresiliente in acciaio dotata di nodi travecolonna a basso danneggiamento e di Nodi di base auto-centranti

Negli ultimi decenni, diversi studi di ricerca hanno focalizzato sensibilmente l’attenzione su sistemi strutturali innovativi, allo scopo di ottenere un comportamento ‘sismo-resiliente’. Con riferimento alle strutture intelaiate metalliche, è abbastanza recente la proposta di impiegare collegamenti dotati di dissipatori ad attrito in grado di conferire all’edificio un comportamento a basso danneggiamento. Però, sebbene siano stati eseguiti in tempi recenti già un certo numero di analisi su connessioni trave-colonna, l’utilizzo di nuove configurazioni di nodi alla base risulta ancora essere un recente sviluppo. I tradizionali nodi di base subiscono grandi deformazioni plastiche e danni, portando così alla necessità di misure di riparazione dopo eventi sismici severi. Per superare tali inconvenienti, un tipo di giunto di base auto-centrante è stato testato e studiato dagli autori, dimostrando l'efficienza del sistema in termini di ricentraggio. In questa prospettiva, presso l'Università degli Studi di Salerno, è stata eseguita una campagna di prove pseudo-dinamiche su una struttura in larga scala dotata tali collegamenti, al fine di valutarne l’influenza sulla risposta sismica globale e locale della struttura. Il lavoro presenta il progetto del provino e dei collegamenti, il setup di prova e la strumentazione e, successivamente i primi risultati sperimentali ottenuti. I risultati hanno dimostrato l'efficacia di tale collegamento nel minimizzare gli spostamenti residui, il danneggiamento locale e nel conferire una migliore capacità di ricentraggio all’intera struttura

Damage-Free Self-Centring Link for Eccentrically Braced Frames = Link ricentrante a basso danneggiamento per strutture con controventi eccentrici

Al fine di promuovere il comportamento resiliente delle strutture in acciaio a seguito di eventi sismici di elevata intensità, molti lavori di ricerca hanno sviluppato sistemi ricentranti in grado di ridurre o eliminare il danneggiamento sismico. In tale contesto, questo lavoro presenta un link ricentrante a basso danneggiamento da impiegare all’interno di sistemi in acciaio con controventi eccentrici. Il sistema impiega barre in acciaio ad alta resistenza post-tese per fornire capacità ricentranti e dissipatori ad attrito che ne assicurano il comportamento a basso danneggiamento. Per investigare il comportamento del dispositivo, la tecnologia proposta è stata applicata ad un sistema sismo-resistente con controventi eccentrici in acciaio a quattro piani progettato secondo l’Eurocodice 8, e modelli numerici sono stati sviluppati in ABAQUS considerando il terzo piano della struttura nelle configurazioni con link convenzionale e ricentrante. I risultati numerici sono poi comparati per mostrare i benefici ottenuti adottando il dispositivo proposto